In the United States, a law for enforcing provision of a tire air pressure alert device mounted on an automobile has been put into effect. As a tire pressure monitoring system (TPMS), a system employing an RFID (Radio Frequency Identification) technique has been proposed. In the system, a transponder (a tire pressure detection device) having a pressure detection function and mounted on a tire communicates by radio with a reader (a control unit) mounted on a vehicle body, while utilizing the RFID technique.
Further, a TPMS for monitoring temperature of a tire, as well as the tire air pressure, has also been proposed. Conventionally, the air pressure and temperature measured by the sensor are digitally encoded by a transponder and sent to the vehicle side reader by means of an RFID technique. The digital code contains identification information for and measured values of air pressure and temperature. The reader obtains the measured air pressure and temperature values from the received digital code.